Thomson, a subsidiary of the FTSE-listed TUI Group, has cancelled all holidays to the terror-hit country for the next week.

TUI joint chief executive Peter Long said: "Our whole organisation is reeling with pain to see the suffering that is taking place in Tunisia with our customers...We are doing our very, very best to look after our guests who are still in Tunisia."

At least five British people have been confirmed among the 39 killed in the gun attack which struck the beach between the Soviva and Imperial Marhaba hotels in Sousse yesterday afternoon, according to UK foreign secretary Philip Hammond. He added that a "high proportion" of the casualties were expected to be British.

Thomson had hundreds of customers at hotels in the Sousse area, and confirmed several of the victims were its customers, but was unable to confirm how many had been killed or injured.

A spokesperson said:

Ten Thomson Airways flights are travelling to Tunisia to bring approximately 2,500 Thomson and First Choice customers home.

We will also be cancelling all Thomson and First Choice holidays to Tunisia for the next week. Amendments can be made to bookings on holidays to Tunisia until 24th July and we are scheduling three extra flights to Cape Verde, Rhodes and Gran Canaria for customers wishing to change their holidays.

Travel agency Thomas Cook, who said it did not have any customers or staff caught in the incident, said it had arranged for additional aircraft to fly to Tunisia and had offered customers with booked trips to Tunisia up to and including July 4 to cancel free of charge.

The UK Foreign Office has warned travellers that some attackers may still be at large and has urged people to stay indoors and not disclose information on social media.

Tourism is a key component of Tunisia's economy, generating around 15 per cent of its GDP.